A wide variety of components for automotive and non-automotive applications are now being fabricated using a process in which one part is cast around a portion of another part. In some cases, the different parts of the component are fabricated using different materials, so as to provide a finished component with desired weight and/or strength characteristics. By way of a few specific and non-limiting examples, an engine cradle is formed by casting aluminum end members around the ends of hollow, steel cross-members, or a torsion beam axle assembly is formed by casting an aluminum trailing arm around an end portion of a steel torsion beam, as described for instance in U.S. Pat. Nos. 7,837,230 and 8,496,258.
A typical process for manufacturing an engine cradle includes covering the open ends of each of the hollow, steel cross-members with an end cap. The covered ends of the steel cross-members are then introduced into a mold of predetermined shape and are held in place. Molten aluminum is introduced into the mold at relatively high pressure and is cooled, so as to cast an end member around the ends of each of the cross-members. The purpose of the end caps primarily is to prevent the molten aluminum from entering and filling the cross-member during the casting process. In order to ensure that the molten aluminum does not enter the hollow cross-member during the casting process, typically the entire length of the mating seam between the end cap and the cross-member is welded. Once the casting step has taken place, an X-ray scan of the casting is carried out in order to verify whether there are any defects in the castings.
Of course, the end caps that are used to cover the ends of the cross-members add weight to the cradle, which results in higher unit costs and leads to lower fuel efficiency in the finished automobile. Further, the end caps are sometimes deformed under the influence of the high pressure that is exerted during the casting process. Further still, the presence of the end caps can create air pockets during an e-coating step, and it may be relatively difficult to drain the excess e-coat from the cross-members since the ends of the cross-members necessarily have no holes.
Another disadvantage of this process is that the ends of the cross-members typically are formed into a cylindrical shape, and they are covered using circularly shaped end caps in order to create a pressure vessel that is able to withstand the pressure exerted by the molten aluminum in the mold. Of course, a cylindrical shape is not necessarily an optimal shape for supporting a load during use.
Additionally, transporting, handling and storing of completed cradles can be cumbersome because of the weight of the completed cradle and also because of its size. Often, specialized equipment is required during handling and transporting of the completed cradles. Furthermore, the completed cradles occupy a relatively large amount of space even though each cradle has a large amount of empty space associated therewith. Of course, in the event that an X-ray scan reveals a defect in one of the two castings in a finished cradle, it is necessary to scrap the entire cradle even if the other casting in the cradle has no defects. This can result in a scrap rate for cradles as high as 10% in some cases.
Other components may be manufactured in a similar way, such as for instance torsion beam axle assemblies, control arms, etc. For instance, each end of a steel torsion beam is covered with an end cap as described above, and each end of the torsion beam is introduced into a mold. Molten aluminum is introduced into each mold at relatively high pressure and is cooled, so that a trailing arm is cast around the each end of the torsion beam. Torsion beams, or control arms, that are formed in this manner also suffer the above-noted disadvantages.
In WO 2008/004715, Ko proposes an alternative arrangement for a torsion beam axle. In particular, the torsion beam axle includes a torsion beam, a plurality of trailing arms made from a material different than that of the torsion beam, and connecting tubes made of a material better than that of the trailing arms with respect to weldability with the torsion beam, the connecting tubes integrally coupled with the trailing arms at one end thereof. Unfortunately, Ko merely provides a schematic illustration of a finished torsion beam axle assembly in cross-sectional view, in which the material of the trailing arm surrounds the one end of the connecting tube and extends through anchoring-slots at the one end of the connecting tube. In this rather fanciful disclosure, Ko neither suggests a suitable process for fabricating the finished torsion beam axle, nor does Ko even appear to contemplate the difficulties that are associated with casting the trailing arm around the one end of the hollow connecting tube. As such, it appears that Ko intended for it to be left entirely to the reader to devise a suitable process for forming the torsion beam axle assembly. Such a process must prevent molten material, which is used to form the trailing arm, from being ejected under pressure from the mold via the hollow connecting tube. In addition, such a process must also prevent filling of the connecting tube by the solidified trailing arm material in the finished product. Of course, this leaves it to the reader to solve a significant problem, requiring simultaneous consideration of complex engineering issues as well as safety issues, manufacturing process issues and economic issues.
It would therefore be desirable to provide a bi-metallic component, such as for instance an engine cradle, a torsion beam axle assembly or a control arm, and a process for manufacturing the same, which overcomes at least some of the above-noted disadvantages.